Why was Gotenks a gamble?

[GreatSaiyaman123]

Goku was sure Gohan and Vegeta could learn the fusion quick enough to fight Majin Boo, and Vegeta does learn fusion pretty quick to fight Broly. Yet Piccolo doubts the boys could learn fusion before Boo destroys the planet?

And even after Goku bargains for Boo to wait 2 days, he still thinks it’s a gamble, but a gamble worth taking because the boys are so talented. If they enough time and the boys are so powerful, what is making Goku so unsure? And why can’t the boys learn fusion within a couple hours? Unless they took huge breaks between attempts, SSJ Gotenks was born within a couple hours.

[theherodjl]

It was a gamble because bad guys inevitably showed up whenever Goku was around, each major villain always being much stronger than the last. Goku might've had confidence that Goten & Trunks' fusion could take on the Fat Boo but historically, bad things would also happen whenever the team got overconfident. Goku simply wasn't absolutely sure if somehow, Boo couldn't(and did) become even more powerful than before and muck up the plan of having their children save the day. That had been done with Gohan and Cell 7 years earlier and that narrowly came close to failure.

[Lionel]

It was a gamble because bad guys inevitably showed up whenever Goku was around, each major villain always being much stronger than the last. Goku might've had confidence that Goten & Trunks' fusion could take on the Fat Boo but historically, bad things would also happen whenever the team got overconfident. Goku simply wasn't absolutely sure if somehow, Boo couldn't(and did) become even more powerful than before and muck up the plan of having their children save the day. That had been done with Gohan and Cell 7 years earlier and that narrowly came close to failure.

I agree with theherodjl's observations here. In addition, despite their advanced power they were still just young impressionable children who could make a mistake if they weren't careful. As we know their lack of discretion allowed Buu to gather his bearings and prepare a counter-strategy for their ace in the hole, Gohan. Gotenks had already proven himself to be an imbecile who goes off and jeopardises their prospects for victory just so he can showboat.

[Koitsukai]

I think it has nothing to do with their talent or power, but with their personalities and mindset.
They are spoiled brats, and Goten was not even in complete control of his ki yet. They had never been in a serious situation before, unlike Gohan, who happened to bottle in his first fight.
Goten was even willing to fight Super Buu after he stomped Egg Chichi, so that's another red flag.
Their childish nature isn't what you'd want in a savior, and it ended up being the reason why Super Buu survived Gotenks. They got drunk with power and wasted a great opportunity. It was just a game for them.

Not to mention he just met these kids and probably spent 10 minutes with them altogether, so they are basically 7 year old strangers with superpowers. If he actually knew them, if he knew how immature they were I wonder if he'd still let them take care of it, or if he'd just turn SS3 and kill Buu.
Also, dancing isn't for everybody, AFAHK, they could've been Elaine Benes type of dancers, and she ain't fusing into a world savior any time soon.

TL;DR: their age, too young and unexperienced to have to deal with the greatest threat ever on their own. Gohan had the entire squad behind him vs Cell.

[Mireya]

It took Goku a lot of time to master it. Maybe he wasn't 100% sure they'd master it in time. Or maybe it has to do with Boo destroying important bits of the planet where the DBs may be located. He may not destroy the whole planet, but may end up destroying a ball. Piccolo even advised when Gotenks was launching his all out attack that destroying one ball would cost heavily. Or maybe it also has to do with Goku not being totally sure of how Boo will behave. Sure, he said Boo was excited to fight in 2 days and said he doesn't think he'll destroy the Earth, but he still didn't offer 100% of certainty. Boo can still be unpredictable. And... Since Goku didn't finish Boo when he could, worse things ended up happening with Boo spawning his evil side and becoming way more powerful. So it can even be a kind of "possible feeling of doom" Piccolo had when they were in their way to fight the androids. When Goku got the senzu with Karin to heal Videl, he agreed with Karin's sentiment that something bad was about to happen. He seems to have some sixth sense for some things.

[Godo]

Gotenks was a gamble and a failure initially IMO, even if Piccolo and the others were impressed.
SSJ Gotenks would unfortunately not be able to muster up any resistance against neither Fat Buu or Super Buu.
The thing that turned things around was that Gotenks miraculously achieved SSJ3 (a form he wouldn't have achieved if Goten and Trunks hadn't seen Goku transform into it, I'm sure).
All in all, it would be a one in a million chance, a gamble which they shouldn't have won.
Also, they had no way to know how strong Gotenks would become initially either.

[ZombieVito]

Goku was sure Gohan and Vegeta could learn the fusion quick enough to fight Majin Boo, and Vegeta does learn fusion pretty quick to fight Broly. Yet Piccolo doubts the boys could learn fusion before Boo destroys the planet?

And even after Goku bargains for Boo to wait 2 days, he still thinks it’s a gamble, but a gamble worth taking because the boys are so talented. If they enough time and the boys are so powerful, what is making Goku so unsure? And why can’t the boys learn fusion within a couple hours? Unless they took huge breaks between attempts, SSJ Gotenks was born within a couple hours.

The kids might be strong but they are complete amateurs.

[GreatSaiyaman123]

Yeah I think the lack of experience answers it all. It's the same reason they boys are kept away the entirety of Super despite being stronger than the humans (And maybe Piccolo, back in RoF). Cockiness seems to be a trait for every fusion, but I think it's unclear if SSJ3 Gotenks could have finished Boo earlier so I wouldn't hold that against him.

Gotenks was a gamble and a failure initially IMO, even if Piccolo and the others were impressed.
SSJ Gotenks would unfortunately not be able to muster up any resistance against neither Fat Buu or Super Buu.
The thing that turned things around was that Gotenks miraculously achieved SSJ3 (a form he wouldn't have achieved if Goten and Trunks hadn't seen Goku transform into it, I'm sure).
All in all, it would be a one in a million chance, a gamble which they shouldn't have won.
Also, they had no way to know how strong Gotenks would become initially either.

But Goku had experience seeing Metamorians fuse in the Afterlife, that's literally why he had the idea to fuse. Fusion makes the fusers some tens of times stronger, and that's more or less where the boys sit in comparison to Boo. It was hardly a one in a million, probably more than 50/50 since Goku had a chance to kill Boo right there. His behaviour here was similar to how he acted in the Cell Games with Gohan, a gamble that also paid off pretty well.